1. Field of the Invention
The present invention relates to a drive control apparatus, an image pickup apparatus, and a drive control method, and more particularly to a technology of performing a micro step drive of a stepping motor.
2. Description of the Related Art
In an image pickup apparatus such as a video camera or a digital camera, a stepping motor is widely adopted as a mechanism for driving a focus lens or a zoom lens. A rotation power of the stepping motor is transmitted to the lens via a transmission mechanism constituted of a screw shaft and a rack engaging with it, and it becomes a power source of the lens in an optical axis direction.
In an AF (autofocus) control of the video camera or the digital camera, a TV-AF method is widely adopted. The TV-AF method detects a signal obtained by extracting a high frequency component in a predetermined range of a taken image as an AF evaluation value to control a movement of the focus lens so that the AF evaluation value is maximized. In the TV-AF method, a repeating motion of a minute movement in both directions of a telephoto side and a wide angle side of the focus lens (hereinafter, referred to as a “wobbling”) is repeated to detect a position of the focus lens where the AF evaluation value is maximized.
As a drive method of the stepping motor for the AF control of the TV-AF method, a micro step drive method is widely adopted. The micro step drive method uses an excitation current having a sinusoidal waveform to realize a drive with steps (micro steps) in which one step (the minimum rotation angle) of the stepping motor is finer.
However, in the micro step drive method, the rotation angle in each micro step varies in accordance with a phase of the excitation current (hereinafter, referred to as a “step phase”). The cause of the variation is that a magnetic attractive force pulsates by a bias of a magnetic flux distribution of a permanent magnet that is a rotor of the stepping motor (hereinafter, referred to as “cogging”). The variation of the rotation angle in each step phase by the cogging causes the variation of movement amplitude of the focus lens at the time of the wobbling operation. As a result, a problem described below may be generated.
When a wobbling width (the number of driven micros steps) is determined on the basis of a step phase where a rotation angle is small, an amount of movement of the focus lens may be equal to or larger than a depth of field in the wobbling operation in a step phase where the rotation angle is large. This causes a float of an in-focus state. On the other hand, the wobbling width is determined on the basis of a step phase where the rotation angle is large, a so-called step-out may be generated since kinetic energy for a minute movement of the focus lens cannot be obtained in the wobbling operation in the step phase where the rotation angle is small.
Japanese Patent Laid-Open No. 01-218393 proposes a correction of a waveform of an excitation current referring to a correction table so that a rotation angle in each step phase becomes constant.
However, for the reason below, the proposal in Japanese Patent Laid-Open No. 01-218393 is not necessarily suitable for the image pickup apparatus such as a video camera or a digital camera. In other words, a bias of a magnetic flux distribution of a permanent magnet has a strong temperature dependency, but a static correction using a correction table effectively functions only at a specific temperature. Therefore, it is not suitable for the video camera or the digital camera that is used in an environment which dynamically changes. When a waveform of an excitation current is corrected, it causes the generation of a motor vibration sound at the time of inversing a rotation direction in a control which reverses the wobbling such as an AF control because a phase difference between stop positions in a positive direction and an inverse direction is generated. Accordingly, the proposal in Japanese Patent Laid-Open No. 01-218393 is not suitable for the video camera or the digital camera that stores a voice.